A project is in progress for a systematic, rigorous, quantitative validation of all Geant4 physics models against experimental data, to be collected in a Geant4 Physics Book. Due to the complexity of Geant4 hadronic physics, the validation of Geant4 hadronic models proceeds according to a bottom-up approach (i.e. from the lower energy range up to higher energies): this approach allows establishin the accuracy of individual Geant4 models specific to a given energy range on top of already validated models pertinent to a lower energy. Results are presented concerning the lower hadronic interaction phases, involving nuclear de-excitation and pre-equilibrium (up to 100 MeV). All Geant4 electromagnetic and hadronic physics models, and pre-packaged physics configurations distributed by the Geant4 Collaboration (PhysicsLists) relevant to this energy range have been included in the validation test. The electromagnetic models are Standard, LowEnergy-ICRU, LowEnergy-Ziegler (Ziegler-1977, Ziegler-1985, Ziegler-2000). The hadronic models for inelastic scattering involve Nuclear De-excitation in two variants (default and GEM), Precompound (with and without Fermi break-up), Bertini and Binary Cascade, and parameterised models. The models for elastic scattering under test are the parameterised one and the newly developed Bertini Elastic. Various prepackaged PhysicsLists are also subject to the same validation process. The validation of Geant4 physics models is performed against experimental data measured with 2% accuracy. The quantitative comparison of simulated and experimental data distributions is performed through a sophisticated goodness-of-fit statistical analysis, including Anderson-Darling and Cramer-von Mises tests. Please note that the speaker name is preliminary; the actual speaker among the authors will be communicated later.